There has conventionally been proposed a technique for enabling high speed printing without increasing current capacity of an adaptor of a thermal head. For instance, in a conventional printer control method, when printing an image, pixel values each corresponding to one line along the main scanning direction of the image of a printing object are accumulated to obtain an integrated value of the pixels of the image. If the integrated value exceeds a reference value, the print speed is set slower than a normal speed. As a result, a large amount of application energy can be secured to a thermal head at one line, so that density level printable at a thermal head can be enhanced, while suppressing the power capacity of the power source supplying exothermic energy to the thermal head as low as possible. Further, if the integrated value is below the reference value, the print speed is set in the normal speed, so that the decrease in the print speed can be avoided and the image can be printed effectively. Accordingly, minimizing the volume (downsizing) of a power source and optimizing the print speed can be realized simultaneously.
However, in the conventional printer control method, whether to set the print speed to be the normal speed or to be slower than the normal speed is determined by each line along the main scanning direction of the printing object image. Accordingly, if the setting of the print speed is changed by each one line with regard to the integrated value accumulated by a pixel unit forming the one line, the setting change occurs so frequently that the print quality is adversely affected.